Drum brakes of the duo servo type are designed to obtain a servo action by means of a wheel cylinder or similar device action on the brake shoes at or near one adjacent pair of shoe ends. They are also designed to obtain additional mechanical servo action once the brake shoes have been moved into engagement with the brake drum friction braking surface by the wheel cylinder. The primary brake shoe, having engaged the drum friction braking surface with the drum rotating, is forced by rotating action of the drum to move arcuately in the direction of drum rotation. The other end of the primary brake shoe from the wheel cylinder acts through the adjuster strut to push on the adjacent other end of the secondary brake shoe, urging the secondary brake shoe arcuately and also outwardly toward the drum friction braking surface with greater total braking force then being exerted on the secondary brake shoe than is obtained by the wheel cylinder acting alone on the that shoe. This provides a multiplication of the brake actuating force caused by the direct action of the wheel cylinder alone, causing the braking action of the brake to increase at a faster rate than a straight-line increase with the increase in wheel cylinder brake apply force. The brake shoe ends at or near where the wheel cylinder is located often have an anchor which serves as a brake shoe end positioning stop when the brakes are not activated, the brake shoes being retracted to this position by retraction springs continually urging the one adjacent pair of shoe ends toward engagement with the anchor. The anchor also transfers the brake forces tending to rotate the brake shoes from the shoes to the backing plate and then to a fixed part of the vehicle.
In recent years it has become more common to provide disc brakes for the front wheels of a vehicle and drum brakes for the vehicle rear wheels. Also, it has become more common to use front wheel drive vehicles, in which the dynamic weight shift during braking loads the front wheel brakes considerably more than the rear wheel brakes. Since the rear wheel brakes do not have as heavy a demand for braking under these circumstances, the rear wheel brakes have either been changed to the leading-trailing type or, if duo servo brakes, a pressure proportioner has been used to proportionately decrease the brake actuating pressure to the rear brakes in relation to the front brakes. This has been desirable because the disc brakes are 1:1 straight-line function brakes in that their braking effort is directly proportional to the actuating pressure being applied to them, while duo servo brakes increase their braking effort at a higher rate than 1:1 as the actuating pressure to them increases. Proportioners prevent unwanted high duo servo drum brake outputs.
However, in braking a forward moving vehicle, which is the usual case, the secondary brake shoe end engaging the brake shoe anchor tends to try to wedge itself between the anchor and the brake drum, riding up on the anchor away from the center of rotation of the drum as it does so. When this occurs, the drum brake output becomes somewhat more erratic and also is higher due to the increased mechanical servo action. The shoe then wears faster, heats up more with a consequent reduction in fade resistance, and brake noises increase. While not all of these results occur in well-designed duo servo brakes, the possibility of any of them occurring should be obviated or diminished substantially if possible.